Roasting, coarsening and hardening of iron sulfide materials

ABSTRACT

A method for converting finely divided oxide bearing material, preferably iron oxide bearing materials to coarse powder form suitable for further metallurgical treatment. The material is agglomerated, preferably by rolling the material between smooth or surface knurled rolls, or by micropelletizing the material, to a particle size distribution suitable for fluidizing purposes. The material is then passed to a fluidized bed furnace, in which it is roasted together with sulphide material at a temperature of between 600* and 1,100* C, the necessary heat being generated by combustion of the ingoing sulphides, whereafter the resulting hardened and coarsened product is removed hot from the bed.

United States Patent [191 Giirling Dec. 17, 1974 ROASTING, COARSENING AND 2,127,632 8/1938 Najarian....., 75/3 HARDENING OF IRON SULFIDE 2,807,534 9/1957 Haley et al 75/3 3,346,364 10/1967 Warnes 75/9 X MATERIALS 3,386,815 6/l968 Gorling et al... 75/9 [75] Inventor: Karl Giiran Giirling, Lidingo, 3,598,569 8/1971 Epperlymj 75/3 Sweden I Primary Examiner-A. B. Curtis- [73] Asslgnee' Bohden Aknebolag Stockholm Attorney, Agent, or FirmStevens, Davis, Miller &

Sweden M osher [22] Filed: Feb. 20, 1973 [21] Appl. No.1 333,480 [57] ABSTRACT A method for converting finely divided oxide bearing Related Application Data material, preferably iron oxide bearing materials to [63] Continuation of Ser. No. 135,116, April 19, 1971, coarse powder form suitable for further metallurgical abandoned. treatment. The material is agglomerated, preferably by rolling the material between smooth or surface Foreign Application Priority Data knurled rolls, or by micropelletizing the material, to a Apr. 20, 1970 Sweden .l 5391/70 Particle Size distribution Suitable for fluidiling P poses. The material is then passed to a fluidized bed [52] US. Cl. 75/3, 75/9 f n c i which it i roasted og r h lphi [51] Int. Cl C20b 1/10 m r l a a mp r ur of etween 600 an 1,l00 [58] Field of Search 75/3, 9 C, the necessary heat being generated y combustion of the ingoing sulphides, whereafter the resulting hard- 1 56] References Cit d ened and coarsened product is removed hot from the UNITED STATES PATENTS l,684,006 9/1928 Bent et al. 75/3 5 Claims, 3 Drawing Figures PATENTEDBEBT 7 $854,931

sniirengz ROASTING, COARSENING AND HARDENING OF IRON SULFIDE MATERIALS This is a continuation of application Ser. No. 135,116, filed Apr. 19, 1971, now abandoned.

The present invention relates to a method for treating finely divided materials containing metal oxides, preferably materials containing iron oxide, so as to convert the material to coarse powder form suitable for further metallurgical treatment.

Certain raw materials which are to undergo metallurgical treatment possess unsatisfactory physical properties. These materials are often in fine powder form and have a low degree of denseness and as a result thereof are highly dust forming. Moreover, such materials are liable to absorb water when moistened. Such properties render the material less suitable for handling, storage, transportation and further metallurgical treatment, such as pan sintering or sintering in conveyor-type furnaces, or treatment in rotary furnaces, shaft furnaces or electric smelting furnaces. These materials are also often too fine for use with fluidizing processes, in which large quantities of gaseous fluidizing medium are charged to the system.

A particularly serious and difficult problem is one concerned with the handling, storing, transporting and processing of materials obtained from the roasting of fine metal sulphides, particularly flotation concentrates, which concentrates are finely ground before being enriched, in order to facilitate selective isolation of the different minerals contained therein. Similar problems exist when treating roasted products derived from coarser sulphide materials which have been decrepitated during the roasting process.

THE PRIOR ART raw materials at elevated temperatures (800 l,l

C). These experiments, however, have not lead to processes which can be used industrially, since it is both technically difficult and expensive to heat extremely fine material and since it has been impossible to find a sufficiently strong material for the briquette molds.

GENERAL DESCRIPTION OF THE INVENTION With the method of the present invention, the finely divided metal oxide bearing material, preferably iron oxide material, is treated to convert it to a coarser form suitable for further metallurgical treatment, the invention being mainly characterized in that the material is agglomerated, preferably by rolling said material between smooth or surface knurled rolls or micropelletized to a particle size distribution suitable for fluidizing purposes, and is then charged to a fluidized bed furnace, in which it is roasted together with sulphide material at a temperature of between 600 and l,l0O C, the necessary heat being generated by ingoing combusting sulphides, whereafter the obtained hardened, coarse product is removed hot from the furnace bed.

The method according to the invention will be described in the following mainly with reference to materials which contain iron oxide, although it will be understood that the method can also be used for treating other types of metal oxide material, for example such as roasted products of copper sulphides, zinc sulphides and iron sulphides.

The agglomerated material charged to the fluidized bed shall be fluidizable and have a maximum particle size which does not substantially exceed 10 mm. Subsequent to being purified, the sulphur dioxide roaster gas formed during the roasting process, which can be obtained at the same high concentrations obtainable with other types of sulphide roasting processes, is utilized in a conventional manner for the manufacture of sulphuric acid or liquid S0 The methods described in Swedish Pat. Ser. No. 304,767, the Belgian Pat. No. 740,320 and the Spanish Pat. No. 340,602 can be used when manufacturing the agglomerate, although it is also possible to produce the agglomerate in a manner known per se, as by micropelletizing according to the method disclosed, for example in Swedish Pat. No. 217,803.

According to Swedish Pat. No. 304,767, finely divided iron containing material to be subjected to metallurgical treatment is agglomerated at elevated temperatures by rolling the material between substantially smooth or surface knurled rolls at a temperature of between 300 and 600 C, whereafter the formed cakes or compacts are crushed.

The Spanish Pat. No. 340,602 describes a method for agglomerating finely divided iron oxide material derived from the roasting of iron sulphides, the iron oxide being intended for use for further metallurgical treatment by suction sintering, smelt reduction (Dored) or with such processes as sponge iron manufacturing processes and chlorinating volatilization processes, hot roasted products being compressed between rolls provided with briquette molds at a temperature of between 200 and 430 C, whereafter the formed briquettes are optionally broken up into smaller pieces.

The Belgian Pat. No. 740,320 describes a method of agglomerating iron oxide material, preferably magnetite, according to which the material is agglomerated while cold or while heated to a temperature of C between substantially smooth or surface knurled rolls in the presence of a lubricant which is added in such quantities thatthe friction between the particles of material is substantially reduced and at most in quantities which can be vaporized by the heat developed by said friction.

According to another known method the material to be pelletized is moistened to a uniform moisture content and passed over a vibrating surface, the material being formed into small pellets which are rolled to nodules having a diameter of 0.5 to 5.0 mm, whereafter the nodules are dried. The moisture content of the agglomerated material should not be sufficient to cause the nodules to disintegrate as a result of the steam generated when charging the nodules to the hot fluidized bed.

The finely divided iron oxide bearing material may consist totally or partially of roasted products obtained from a previous sulphide roasting process. Thisroasting process and roasting effected to harden the agglomerates may be expeditiously carried out in the same furnace in a manner whereby the finely divided sulphide material to be roasted is charged to a fluidized bed furing, for example, one of the aforementioned methods.

The agglomerated roasted products are then returned to the fluidized bed furnace and heated by the reaction heat generated upon combustion of the fine sulphide material. Since practically all the fine powder material accompanies the roaster gases, the material removed from the bed will be in the form of a coarse powder and will contain only a very small quantity of fines.

If roasting and hardening are effected in the same furnace, a surplus of heat is normally obtained during combustion of the sulphide material, which can either be cooled by steam-generating cooling elements located in the bed or may be utilized in the furnace for heating other material. This latter material, prior to the agglomerating stage, is suitably mixed with the fine roasted products removed from the furnace.

Even though it is often to advantageto conduct the sulphide material roasting process and the roasting process in which the material is hardened in the same furnace, it may be desirable to perform the first mentioned process in one or more, separate furnaces and the latter process in a special furnace, to which all material is charged from said separate furnaces. Heat for this latter roasting process is obtained by combusting either separately charged fine or coarse sulphide material or the iron sulphides present in the iron oxide agglomerates. Heat generated during the process can be recovered in a conventional manner with cooling coils located in the fluidized bed, or through a waste heat boiler or like apparatus, either before or after the finegrain products entrained with the roasted products have been separated therefrom. If this separation is effected, for example, in hot cyclones before the roaster gases have been cooled in the waste heat boiler, the heat in the separated finely divided roasted products may also be recovered; for example, in a fluidized bed cooler provided with steam generating cooling elements for example. If it is desired to reduce the amount of sulphide material which must be charged to the roasting process conducted to harden the material, the heat removed with the roaster gases can be used to preheat the air charged to the fluidizing furnace. it is, of course, also possible to beat this air by burning gas or oil.

Binding agents such as bentonite, lime, sulphite waste liquor or fine powder sponge iron can be added when agglomerating certain kinds of finely divided materials.

The method of the present invention is particularly suitable for treating iron oxide material obtained by roasting iron sulphides, such as pyrites and pyrrhotite.

The conditions in the roaster furnace can be adjusted in connection with hardening of the agglomerates so that magnetite is obtained. The material entrained with the roaster gases can then be magnetically enriched. Such a roasting process is described in the Swedish Pat. No. 204,002. The roasting process according to the present invention can also be used to advantage in combination with the processes described in the Canadian Pat. No. 796,672. According to this patent, iron sulphide material, for example, is roasted in a fluidized bed furnace at temperatures ranging between 700 and l,l C, while oxygen gas or gas containing oxygen is charged in such quantities that the partial pressure of oxygen in the resulting roaster gases is maintained below a pressure temperature curve (ll in FIG. 1), in a coordinate system, where the ordinate expresses the partial pressure of oxygen in the atmosphere as log P and the abscissa the temperature in C, which passes through the following points:

log P Temperature but not below a corresponding curve (Ill) passing through the following points:

log P Temperature l5.0' 700 13.5 800 it? a i0I0 1050 whereby the material is freed from sulphur and sulphur compounds so that cinders substantially free of sulphur are obtained.

According to the same patent, arsenic bearing iron sulphide materials can be roasted in a similar manner, the partial pressure of oxygen being maintained beneath an analogically drawn pressure temperature curve (I) passing through the points:

log P Temperature whereby secondary reactions between arsenic and its compounds and the roasted products are counteracted and the latter is obtained in a condition in which it is substantially free from sulphur and arsenic.

With a practically preferred embodiment, the partial pressure of oxygen is maintained in the instance just mentioned beneath a pressure temperature curve (IV) which passes through the following points:

log P Temperature tion of arsenic and sulphur compounds and the reformation of arsenate in connection with the subsequent afterburning stage. Since with the present inven tion all roasted products are ultimately removed from the bed, in which suitable temperatures and furnace atmosphere can be adjusted without undue difficulty, it is not necessary to separate the fine roasted products entrained with the roaster gases at high temperatures, and consequently neither is it necessary to separate the products in hot cyclones, which are expensive to manufacture and service. The roasting process can thus be effected, with minor modifications, in the same type of BASF-furnace as that used with oxidation roasting processes.

ADVANTAGES Important advantages are afforded by the method of the present invention. Thus, a fine powder material can be converted to a coarse powder roasted product which is practically totally free from all fine material by windsifting the roasted product in the bed. The velocity at which the gas is injected in the bed determines the smallest particle size of the material removed from the bed. With conventional gas velocities and load, it is thus possible to produce without difficulty in a fluidized bed furnace a roasted product which practically completely comprises material having a particle size in excess of 0.2 mm.

The method is particularly suitable for roasted products obtained from flotation dressed minerals, which when subjected to conventional roasting processes often result in a too fine product.

In normal roasting processes, the fineness of the flotation concentrate also makes it difficult to retain the material in the bed for a length of time sufficient to obtain more complete conditions of equilibrium. This disadvantage is completely eliminated with the roasting process of the present invention.

The roasting process of the present invention also affords the advantage whereby the roasted product obtained is practically dust-free and can be transported and handled without being moistened, as is generally necessary in the case of finely divided roasted products. Because of the particle size distribution of the material, it is well suited for suction -sintering processes or for further treatment in, for example, rotary furnaces, multihearth furnaces or shaft furnaces. The material is particularly suited for further treatment in fluidized bed furnaces. Another advantage afforded by the present invention is that the material can be removed from the bed in a hot condition and used directly for further treatment in other processes.

It has been found that the products obtained by means of the method of the present invention are suitable for chlorinating volatilization, for example according to French Pat. No. 1,570,317 or the DDR-Patent No. 70,609, in which oxidic iron material containing one or more of the substances Cu, Zn, Pb, Co, Ni, Au, Ag, As, Bi, Sb, and S is treated in a hot condition, for example, at temperatures of between 600 and 800 C with a gas containing chlorine or chlorine compounds as hydrochloride or with material which give off a gas containing chlorine or chlorine compounds, thereby volatilizing the said substances. The chlorinating process can be effected in different types of furnaces. Furnaces having a fluidized bed or moving bed have been found particularly suitable.

Repeated roasting of the roasted product advantageously affects its chemical composition, particularly'in the case of magnetite yielding roasting. The roasted product is retained in the furnace for a very long period of time, the average stay time of the product in the furnace being of the order of hours, while the stay time with conventional flotation pyrites roasting processes is of the order of only seconds. Thus, when applying the method of the present invention it is possible to use a considerably smaller roasting area and furnace volume per unit of roasted concentrate. The method is particularly suitable for roasting flotation concentrate and other finely divided sulphide material, such as pyrites and pyrrhotite. Whereas conventional magnetite yielding roasting processes do not generally permit such a high load on the roasting area and the furnace volume with oxidation roasting when applying the method of the present invention the load may be even higher when roasting to magnetite than with the normal oxidizing roasting of flotation concentrates. A low sulphur content can be obtained and if the raw material contains such substances as arsenic, lead, antimony, bismuth or tin these may be isolated very effectively.

Another advantage afforded by the method of the invention is that either magnetite or hematite can be obtained as the final product, depending upon the purpose for which the material is intended. Magnetite is generally to be preferred, although in certain instances hematite is desirable.

Coarse grain pyrite cinders and other iron oxide bearing mineral products which are to be dressed magnetically normally require extensive crushing. This results in a product of such fineness that it is unsuitable for further treatment. In accordance with the invention, if the roasting is a magnetite yielding roasting process or if the starting material is a magnetitic material, the fine material is subjected to magnetic separation, agglomerated and removed from the hardening stage in the desired enriched coarse powder form.

If, for the purpose of increasing the capacity of an existing plant for manufacturing sulphur dioxide gas, it is desired to build an addition furnace, it is possible, if the roasting method of the present invention is applied, to use a simpler furnace for roasting in connection with hardening than with conventional roasts, since it is possible to exclude the arrangement of cooling coils in the bed, owing to the fact that the temperature can be regulated by adding cold solid material. It is normally necessary to separate the roasted products from the gas prior to cooling the gas, in order to avoid condensation and arsenate formation; this is particularly true when roasting to expel arsenic in accordance with the aforementioned patents. This separation is effected in hot cyclones. ln contradistinction to this, if the arsenic is roasted off in accordance with the present invention, it is not necessary to use hot cyclones, since the final product is removed hot from the bed. If the roasted products entrained with the roaster gases are cooled in a waste heat boiler, it is not necessary to cool the products in a fluidized bed cooler prior to subjecting the products to a possible magnetic enrichment process and agglomeration.

As will be evident from the aforegoing, the method of the present invention is not solely a novel agglomerating method, but can be characterized equally as well as a new roasting method.

The invention will now be illustrated with reference to FIGS. 2 and 3. In the embodiment illustrated in FIG. 2, the roasting process is effected in two separate furnaces, whereas in the embodiment of FIG. 3 the previous sulphide roasting process and roasting in connection with hardening are both effected in the same furnace.

FIG. 2 illustrates a fluidized bed furnace 1, to which finely divided sulphide material is charged from a feed ing means 2. Air is supplied through a line 3. Roasting is effected in a manner whereby all the roasted prodnets are removed with the roaster gases through a line 4. The reference numeral 5 indicates a line, shown with dotted lines, through which roaster gases containing roasted products obtained from other, similar furnaces can be charged to the process. The reference numeral 6 indicates an afterburning zone. When roasting has been effected with such limited quantities of roaster air that considerable quantities of elemental sulphur are present in the roaster gas, the after-burning stage is carried out in zone 6. The roaster gas is removed from the zone 6 and passed through a waste heat boiler and optionally through an additional after-burning zone 8. Any roasted products falling out in the waste-heat boiler 7 areconveyed directly to the agglomerating stage 10 via a line 9. The roaster gas is passed from the waste heat boiler 7 to a cyclone 11, in which the entrained roasted products are separated. The roaster gas, freed from roasted products and now containing sulphur dioxide and other waste gases, is removed for optional use in the manufacture of sulphuric acid or liquid sulphur dioxide. The gas is suitably purified for this purpose in an electrofilter 12. The separated, entrained roasted products are removed from the cyclone 11 and passed to a system of agglomerating rolls 10. The products are than passed to a further fluidized bed reactor 13, in which they are roasted once more. Air is supplied via a line 14. If the returned agglomerated products do not contain sufficient sulphur material, more oxidizable sulphide material can be charged from a supply locker 15. The roaster gases from the furnace 13 are removed via a line 16 and joined with the roaster gases arriving from the furnace 1. Fine material from the roaster gases is separated in the cyclone 11 and agglomerated between the rolls l0, whereafter it is returned to the furnace 13, while coarse material is discharged as an end product and passed to possible further processes via a line 17.

With the embodiment illustrated in FIG. 3, roasting and hardening are effected in a fluidized bed furnace 18. Fine iron sulphide material is charged to the furnace 18 through a supply means 19. Fine hematitic iron oxide material to be magnetically enriched may be charged from a supply means 20. Departing roaster gases and fine material entrained therewith are cooled in a gas cooler 21, which in the exemplary embodiment is in the form of an apparatus for pre-heating the roasting air applied to the furnace 18 through a line 22. After the cooling stage, the roaster gas is passed to a cyclone 23, in which entrained roasted products are separated. The purified roaster gas is removed via a line 24. If it is necessary to enrich the material to be roasted in accordance with the invention, the roasting process in furnace 18 is adjusted so as to produce magnetite. The roasted products separated in the cyclone 23 are passed to the magnetic enriching means 25. Other finely divided magnetitic material can also be passed to the means 25 through a supply means 26. The obtained magnetic concentrate is then passed to a system of rolls 27 to be agglomerated. Waste material is removed via a line 28. Other finely divided products can also be passed to the agglomerating stage, via the supply means 29. The agglomerated products are passed to the furnace 18, via a line 30, to be hardened and wind-sifted. The product, which is in the form of a coarse powderlike roasted material, is removed via a line 31 and optionally passed to other stations for further processing.

The invention will now be illustrated by means of the following example.

EXAMPLE 1,600 kg of flotation pyrites were roasted each hour in a fluidized bed furnace, in which the bed was cooled by steam generating cooling elements arranged therein. Practically all the roasted products derived from the flotation pyrites were removed from the furnace with the roaster gases and separated in a cyclone. The roasted products were then cooled in a fluidized bed cooler to a temperature of 200 C and on a cooling redler to a temperature of C and moistened in a drum to a 2 percent water content. The material was rolled in this condition between smoothed rolls and returned to the furnace for repeated roasting and to be hardened. The roasted products were windsifted during the repeated roast. Approximately 10-15 percent of the rolled material was herewith entrained with the roaster gases and returned. to the agglomerating state via the cyclone together with the roasted products derived directly from flotation pyrites from the previous roasting stage. Roasting was effected in a manner whereby practically all material was converted to magnetite in the earlier roasting stage. Since the roasted products obtained thereform consisted almost exclusively of iron oxide, it was not necessary to further enrich the product magnetically. The starting material (1,600 kg/h) contained 51 percent sulphur and 0.4 percent arsenic, while the end product (approximately 1,000 kg/h) contained roughly 0.1 percent sulphur and 0.01 0.02 percent arsenic. All material having a particle size smaller than 0.2 mm was blown off, whereby the end product removed from the bed was totally in the form of a coarse powder product. Screen analysis of the input and output material are shown in the following table:

In other tests, roasted products from different furnaces have been combined, agglomerated and roasted, wherewith the same good results with respect to sulphur and arsenic expulsion were obtained. When the previous sulphide roasting process and roasting in connecting with hardening were conducted so as to form hematite, good sulphur expulsion results were obtained, while the expulsion of arsenic was unsatisfactory.

What I claim is:

1. Process for the production of a hardened, coarse roasted product from iron sulphide, characterized in supplying the iron sulphide to the bed of a fluidized bed furnace where the iron sulphide is roasted and substantially the whole amount of roasted goods is entrained by the roasting gas,

that the roasted fine grained product is separated from the gas in a gas cleaning apparatus,

that the separated material is agglomerated to a particle size distribution suitable for fluidizing,

that the agglomerates formed in a substantially dry state are charged to said roasting furnace and therein subjected to hardening and further roasting under influence by the heat developed at said roasting of the iron sulphide at a temperature of between 600 and 1,l C, and

that the hardened roasted and substantially sulphurfree agglomerates are removed from the bed.

2. Process according to claim 1 characterized in that together with agglomerates formed of separated, roasted material from the fluidized bed furnace, other agglomerates of iron oxide material are simultaneously hardened in the fluidized bed.

3. Process according to claim 1 characterized in introducing finely-divided iron oxide material together with the iron sulphide and controlling the conditions in the roasting furnace to obtain magnetite.

4. Process according to claim 1 characterized in introducing arsenic containing iron oxide material together with the iron sulphide and controlling the conditions in the roasting furnace to expell arsenic.

5. Process according to claim 1 characterized in supplying to the roasting furnace iron oxide material which during the iron sulphide roasting simultaneously is thermally treated. 

1. PRROCESS FOR THE PRODUCTION OF A HARDENED, COARSE ROASTED PRODUCT FROM IRON SULPHIDE, CHARACTERIZED IN SUPPLYING THE IRON SULPHIDE TO THE BED OF A FLUIDIZED BED FURNACE WHERE THE IRON SULPHIDE IS ROASTED AND SUBSTANTIALY THE WHOLE AMOUNT OF ROASTED GOODS IS ENTRAINED BY THE ROASTING GAS. THAT THE ROASTED FINE GRAINED PRODUCT IS SEPARATED FROM THE GAS IN A GAS CLEANING APPARATUS, THAT THE SEPARATED MATERIAL IS AGGLOMERATD TO A PARTICLE SIZE DISTRIBUTION SUITABLE FOR FLUIDIZING, THAT THE AGGLOMERATES FROMED IN A SUBSTATIALLY DRY STATE ARE CHARGED TO SAID ROASTING FURNACE AND THEREIN SUBJECTED TO HARDENING AND FURTHER ROASTING UNDER INFLUENCE BY THE HEAT DEVELOPED AT SAID ROASTING OF THE IRON SULPHIDE AT A TEMPERATURE OF BETWEE 600* AND 1,100*C, AND THAT THE NARDENED ROASTED AND SUBSTANTIALLY SULPHUR-FREE AGGLOMERATES ARE REMOVED FROM THE BED.
 2. Process according to claim 1 characterized in that together with agglomerates formed of separated, roasted material from the fluidized bed furnace, other agglomerates of iron oxide material are simultaneously hardened in the fluidized bed.
 3. Process according to claim 1 characterized in introducing finely-divided iron oxide material together with the iron sulphide and controlling the conditions in the roasting furnace to obtain magnetite.
 4. Process according to claim 1 characterized in introducing arsenic containing iron oxide material together with the iron sulphide and controlling the conditions in the roasting furnace to expell arsenic.
 5. Process according to claim 1 characterized in supplying to the roasting furnace iron oxide material which during the iron sulphide roasting simultaneously is thermally treated. 